Project Summary [unreadable]We have localized the archetypal ventralizing morphogen, Sonic hedgehog (Shh), to the late-acting dorsal organizing center of the hindbrain, the choroid plexus. During early embryogenesis, Shh is robustly expressed in the ventral organizing centers referred to as the notochord and floor plate, yet excluded from the dorsal organizing centers called the ectodermal ridge and the roof plate. Because the choroid plexus is thought to supplant the roof plate as a dorsal source of patterning signals during late embryogenesis, and because the onset of Shh expression in the choroid plexus coincides with this interchange, it is appealing to consider that the late-stage embryo re-deploys Shh, but now to act in a dorsal organizing center to influence the development of late-generated dorsal cell types. The studies in this proposal are intended to address the functional significance of Shh produced by the dorsal hindbrain - specifically, by the 4th ventricle choroid plexus. The role of Shh will be examined in terms of its involvement in choroid plexus development itself and of its effects, direct and/or indirect, on dorsal hindbrain cell types. We are employing both conditional loss- and gain-of-function strategies, coupled with comprehensive phenotypic analyses (molecular, cellular, physiological) to address the following aims: Aim 1: To identify Hedgehog (Hh) expressors and respondents in the dorsal hindbrain. Aim 2: To determine the molecular and cellular consequences following removal of Shh specifically from the choroid plexus (CP). Aim 3: To determine whether up-regulation of Shh signaling in specific CP cell types affects CP and/or hindbrain development in accord with that predicted by the conditional loss-of-function phenotype. Relevance - In addition to its role as a source of signaling molecules requried for hindbrain development, the choroid plexus achieves a host of functions essential to the developing brain: production and detoxification of cerebrospinal fluid (CSF), including production of growth factors and hormones essential for brain development and health;removal of waste metabolites, foreign particles, and plaques associated with neurodegenerative diseases;service as the blood:CSF barrier;and proposed capacity for neuro- regeneration and repair of damaged brain tissue. By examining the consequences incurred by the choroid plexus and hindbrain structures following genetic removal of Shh specifically from the choroid plexus, we hope to shed light on genetic programs enacted during normal development of the vertebrate brain, which may present therapeutic strategies for the diseased or aging brain.